Roller finger followers (RFF) are widely used in overhead cam internal combustion engines to sequentially open and close the cylinder intake and exhaust valves. In a typical application, the RFF serves to transfer and translate rotary motion of a cam shaft lobe into a pivotal motion of the RFF to thereby open and close an associated valve.
It is known that, for a portion of the duty cycle of a typical multiple-cylinder engine, the performance load can be met by a functionally smaller engine having fewer firing cylinders, and that at low-demand times fuel efficiency can be improved if one or more cylinders of a larger engine can be withdrawn from firing service. It is also known that at times of low torque demand, valves may be opened to only a low lift position to conserve fuel, and that at times of high torque demand, the valves may be opened wider to a high lift position to admit more air/fuel mixture or air. It is known in the art to accomplish these valve actuations by de-activating a portion of the valve train associated with pre-selected cylinders in any of various ways. One way is by providing a special two-step RFF having a variably activatable and deactivatable central slider or roller which may be positioned as needed for contact with a high lift lobe of the cam shaft. Such a two-step RFF typically is also configured with a pair of rollers disposed at each side of the slider for contact with low lift lobes of the cam shaft on either side of the high-lift lobe. Thus, the two-step RFF causes low lift of the associated valve when the slider of the RFF is in a deactivated (lost motion) position, and high lift of the associated valve when the slider of the RFF is latched in an activated position to engage the high lift lobe of the cam shaft.
One such two-step RFF known in the art is disclosed in U.S. Pat. No. 6,755,167 B2, issued Jun. 29, 2004, the relevant disclosure of which is incorporated herein by reference. In this roller finger follower, an elongate body having first and second side members defines coaxially disposed shaft orifices. A pallet end and a socket end interconnect with the first and second side members to define a central slider aperture and a latch pin channel. The socket end is adapted to mate with a mounting element such as an hydraulic lash adjuster, and the pallet end is adapted to mate with a valve stem, pintle, lifter, or the like. A slider for engaging a high-lift cam lobe is disposed in the slider aperture and has first and second ends, the first end of the slider being pivotally mounted to the pallet end of the body and the second end defining a slider tip for engaging an activation/deactivation latch. The latch pin is slidably disposed in the latch pin channel, the latch pin having a nose section for selectively engaging the slider tip. A spool-shaped roller comprising a shaft and opposed roller elements fixedly attached to ends of the shaft is rotatably disposed in the shaft orifices, the roller being adapted to follow the surface motion of two outboard low-lift cam lobes. Preferably, the shaft is journalled in roller or needle bearings which extend between and through both the first and second shaft orifices.
A drawback of such a roller finger follower is that the latching pin can inadvertently be partially engaged with the slider when the slider is at initial stage of lost motion. The resulting forces between the slider and the latching pin can exceed the hydraulic force available to hold the latch pin position, resulting in the latching pin being ejected (retracted) into the bore in the finger follower. This event results in undesirable noise, wear and error in the calculation of the needed amount of fuel required for a stoichimetric air fuel mixture if the election occurs at a high valve lift position.
It is an object of the invention to improve component durability by controlling the time available during a cam rotation cycle for the slider locking mechanism to transition between its extreme positions.